In the end, the first real challenge to the authority of a new US president came not from a mullah in Iran, a Talib in Pakistan or a Russian hankering to recreate the Soviet Union. It came from a country which can neither feed nor heat itself, and whose leader is thought to be recovering from a stroke. North Korea yesterday sent a rocket hurtling over the Pacific and, if the state-run Korean Central News Agency is to be believed, an experimental satellite is at this moment transmitting the immortal revolutionary songs of Kim Il-sung and Kim Jong-il.

This singing circling loudspeaker probably only exists in Pyongyang's mind. South Korea and the US said that the first stage of the rocket fell into the waters between Korea and Japan, while the other two stages and its payload landed in the Pacific. If true, this would be the second time the Taepodong-2 missile has failed.

Nevertheless, it is the thought that counts. The launch was a violation of two UN security council resolutions, a provocation to Japan over whose airspace it travelled and a slap in the face to President Barack Obama, who, only hours later in Prague, preached the virtues of disarmament. It is hard to know which is sicker - the country or its 67-year-old leader - but from Pyongyang's point of view this game of rocket brinkmanship will almost certainly work, as it did three years ago. Mr Kim jerked George Bush into life by testing a nuclear device in 2006. As a result, the US lurched from a policy of isolating the reclusive regime to one in which it successfully engaged with it. The device this time is different, but its purpose is similar. It is to make Mr Obama sit up and pay attention on his first international tour as president, and put North Korea higher up his to-do list. Which he will almost certainly now do.

The military significance of the launch is more debatable. The rocket fired yesterday could reach Alaska - in theory. In practice it failed. North Korea has already got a tried-and-tested platform with which to deliver its nuclear payloads - both short- and medium-range rockets, one of which can reach Guam. So yesterday's launch does not change the balance of forces or extend North Korea's range. Nor does the launch make it more likely that Iran or Syria, two past purchasers of North Korean rockets, will be rushing to buy this rocket.

The political boost it gives North Korea's leadership is clearer to see. The Dear Leader is due on Thursday to make his first public appearance for nearly a year, having suffered a stroke so serious it prevented him appearing at a parade celebrating the regime's 60th anniversary. Mr Kim now has more cards to play in the stalled six-party talks over its nuclear disarmament.

Progress over denuclearisation has been slow. North Korea eventually produced a 60-page document in English saying it had developed 30.8 kilos of weapons-grade plutonium - enough for between five and seven bombs - in which it addressed US "concerns" over uranium enrichment. Last October Mr Bush took the country off its terrorism list. But progress since then has stalled over US demands for verification. Nor has it been helped by the election of a president in South Korea who has torn up the sunshine policy of his predecessors and demanded to see tangible signs of progress before his country spends more on aid for the north. Shooting a missile raises the price of a settlement. It is vital that Mr Obama stays cool and focuses on the need to keep China onside. Beijing, which reacted coolly to the launch, remains the key to making North Korea's military dictatorship see sense. When the six-party talks restart, as indeed they must, each side will be tempted to demand goodies up front. This must be resisted. For the talks to succeed, they must create a more balanced and sequenced set of actions which allow Pyongyang to surrender its plutonium.